1. Field of the Invention
The present invention relates to dual chamber, rate-responsive pacemakers.
2. Description of the Prior Art
Dual chamber pacing modes have been widely adopted for pacing therapy. Among the dual chamber operating modes is the "DDD" mode, which can pace an atrium and a ventricle, senses both the atrium and the ventricle, and can either inhibit or trigger pacing stimuli for both chambers. This mode has a sensor augmented variant mode called "DDDR", where the "R" stands for rate-adaptive or rate modulation.
A DDD pacemaker includes an atrial sense amplifier to detect atrial depolarizations of the heart, and a ventricular sense amplifier to detect ventricular depolarizations of the heart. If the atrium of the heart fails to beat within a predefined time interval (atrial escape interval), the pacemaker supplies an atrial stimulus to the atrium through an appropriate lead system. Following an atrial event (either sensed or paced) and an atrioventricular (A-V) interval, the pacemaker supplies a ventricular pacing stimulus to the ventricle through an appropriate lead system, if the ventricle fails to depolarize on its own. Pacemakers which perform this function have the capability of tracking the patient's natural sinus rhythm and preserving the hemodynamic contribution of the atrial contraction over a wide range of heart rates.
Many patients have an intact sinoatrial (SA) node, but inadequate AV conduction. For these patients, the DDD mode, which attempts to pace the ventricles in synchrony with the atria, is generally adequate for their needs. Patients with Sick Sinus Syndrome (SSS) have an atrial rate which can either be sometimes appropriate, sometimes too fast, and sometimes too slow. For SSS patients, the DDDR mode provides some relief by pacing the atria and ventricles at a sensor rate determined by a sensor which senses a physiological indicator of the patients' metabolic needs. However, sensor rates are sometimes too high and sometimes too low for a variety of reasons, including, errors related to the input of programmable parameters, limitations of the sensor's ability to accurately sense the physical quantity being sensed, and limitations or problems with the algorithm used to determine the sensor rate.
In addition to the above problems, pacemakers operating in prior art sensor driven pacing modes pace at the sensor rate (and overdrive the atrium) whenever the sensor rate exceeds the sinus rate, even when the sinus rate is actually appropriate, and even when the difference between the two rates is too small to provide any discernible benefit in pacing at the higher sensor rate. Inappropriate sensor rate pacing can lead to unnecessary overdrive of the atrium, and unwarranted expenditure of battery energy.
In commonly assigned U.S. Pat. No. 5,144,949, incorporated herein by reference in its entirety, a DDDR pacemaker is disclosed wherein the operating or pacing mode is switched between the DDD, DDIR and VVIR modes as a function of the difference between the Average Atrial Rate (AAR) and the Average Combined Sensor Rate (ACSR). The DDD pacing mode (which is not a sensor driven pacing mode) is preferred as long as the AAR and ACSR are within a user selected range, and the AAR does not exceed the Atrial Upper Rate Limit (AURL) or is not irregular at higher rates. When significant atrial tachyarrythmias are detected, the VVIR mode is preferred to avoid inappropriate atrial tracking, atrial competitive pacing and risk of inducing more atrial arrhythmias. During episodes of sinus bradycardia and chronotropic atrial incompetence, the atrial rate is not adequate, so the DDIR mode is preferred. The rise up and fall back pacing rates are smoothed by taking place over several heart beats or a predetermined time interval during mode switch transitions.
The mode switching depends on the comparison of the AAR and ACSR up to the AURL and the Sensor Upper Rate Limit (SURL). Generally, when the AAR is within a first range exceeding the ACSR and both are below the AURL and the SURL, DDD pacing is maintained. When the AAR rises out of that range, then the mode is switched to VVIR. Similarly, when the ACSR is within a second range exceeding the AAR, DDD pacing is also maintained. When the second range is exceeded by the ACSR, the mode is switched to DDIR.
When in the DDIR mode, the pacing rate is limited by the SURL, and the mode is switched to DDD when the Instantaneous Atrial Rate (IAR) equals the ACSR. In the DDD mode, the pacing rate is limited to the AURL, and the mode is switched to VVIR when the ACSR falls below the SURL while the AAR remains above the AURL.
In the `949 patent, the first and second ranges are derived as functions f(A-S) and f(S-A) of the AAR and ACSR between the minimum rate and the AURL and SURL and are programmable by the physician. Thus, the actual instantaneous ranges vary as a function of the current AAR and ACSR values.
The `949 patent emphasizes the desirability of maintaining DDD pacing at an atrial escape or A--A escape interval derived from the AAR whenever possible. That is, the DDD mode is forced and departed from only under the conditions described above. The instantaneous pacing escape interval/rate are determined by the measured intervals between atrial events used to formulate the AAR while in the DDD operating mode. The ACSR is also derived independently and only resorted to to set the pacing escape interval/rate in the modes where atrial synchrony is departed from under the above described conditions.
In further U.S. Pat. No. 4,856,523, a rate-responsive pacemaker with automatic mode switching and/or variable hysteresis rate is described. A physiological, activity related, sensor develops a physiologic escape interval/rate between a minimum and a maximum rate limit (SURL) as shown in FIG. 3A thereof. A variable hysteresis escape interval setting a hysteresis rate below the physiologic rate is defined as shown in FIGS. 3B and 4 thereof. In a single chamber, rate-responsive mode, e.g. VVIR mode, if the prevailing V--V escape interval times out and a V-PACE pulse is delivered, then the physiologic escape interval is employed as the next V--V escape interval. If a V-SENSE event occurs in the prevailing escape interval, then the variable hysteresis escape interval is used as the next escape V--V interval. Use of the variable escape interval continues up to the maximum rate which is the SURL less the maximum hysteresis rate increment as shown in FIG. 3B.
The `523 patent emphasizes the importance of switching the pacing mode from the prevailing dual chamber pacing mode to a single chamber mode (typically VVIR) at high pacing rates because the contribution of the atria to cardiac output diminishes at high heart contraction rates while the energy consumption demanded by dual chamber pacing increases. In the dual chamber DVIR mode described and depicted in FIG. 6 thereof, the V-A interval is selected to be either the physiologic escape interval, or the hysteresis interval depending on whether or not a V-SENSE event is detected in the A-V interval. At high rates, the mode is preferably switched to the VVIR mode.
In a further U.S. Pat. No. 5,074,304, a dual chamber, rate-responsive pacemaker is described which correlates the atrial upper rate limit, designated HSR in this patent, to the sensor rate designated KA to allow upper rate tracking to vary with the physiologic sensor rate.
Despite these advances in dual chamber, rate responsive pacemakers, a need exists for a simple system that reliably allows the synchronized pacing rate to preferentially track the underlying sinus rate even if the sensor-derived pacing rate indicates a higher rate, where the underlying sinus rate is near to the sensor-derived pacing rate, so that the DDDR pacing mode is maintained.